It is known to provide a heavy goods vehicle including a tractor and a trailer or semi-trailer with a stability control system which is configured to detect when there is a risk that the vehicle will rollover, and to initiate a stability control intervention, which typically includes the automatic application of the vehicle brakes, in order to reduce the vehicle speed sufficiently to minimise the risk of rollover occurring. Such a stability control system is disclosed in U.S. Pat. No. 6,176,555, and in this system, if the lateral acceleration of the vehicle exceeds a predetermined amount, a test braking pulse is applied to the wheels of the vehicle and the vehicle ABS system monitored. If application of the test braking pulse causes an ABS control intervention to be initiated, it is deduced that one or more of the vehicle wheels has lifted off the ground, and therefore rollover is likely. A stability control intervention, such as automatic actuation of the vehicle brakes, is then initiated. It will be appreciated, therefore, that in order to determine when to apply a test braking pulse, the vehicle must be provided with a sensor capable of determining the lateral acceleration of the vehicle.